The present invention relates generally to a method and apparatus for feeding magnetic powder (hereinafter sometimes referred to as xe2x80x9cpowder feedingxe2x80x9d) into a pressing apparatus, a method for forming a magnetic powder compact, and a method for manufacturing a magnet using the magnetic powder feeding method and the powder feeding apparatus.
An R-Fe-B type rare earth alloy magnet as a representative high-performance permanent magnet (where R denotes an element or combination of elements selected from the group consisting of rare earth elements and yttrium (Y)), Fe denotes iron, and B denotes boron) has a structure including a major phase of ternary system tetragonal crystal compound (R2Fe14B phase) and an R-rich grain boundary phase, and exhibits excellent magnetic properties. The Rxe2x80x94Fexe2x80x94B type magnet is now used in wide fields from various electric appliances for domestic use to peripheral devices for a mainframe computer. There are strong demands towards miniaturization, weight reduction, and advanced performance, so that an Rxe2x80x94Fexe2x80x94B type permanent magnet with much higher performance is desired.
In order to increase the residual magnetic flux density (Br) of an Rxe2x80x94Fexe2x80x94B type sintered magnet, the following are required: (1) to increase a total volume fraction of ferro-magnetic R2Fe14B phases, (2) to make a density of sintered compact closer to a theoretical density of the major phase, and (3) to align an axis of easy magnetization of the major phase crystal grains.
When magnetic powder is to be fed into a cavity (a powder compacting space) of a pressing apparatus, conventionally, a feeder box (or a feeder shoe) is slid onto the cavity, and the powder is gravity-fed from the feeder box into the cavity.
FIGS. 1A to 1C schematically show a conventional powder feeding method by means of a feeder box. According to the conventional method, as shown in FIGS. 1A to 1C, when a feeder box 13 slides in a transverse direction over a cavity 12 constituted by a die 10 and a lower punch 11 of a powder pressing apparatus, magnetic powder 14 in the feeder box 13 is filled in the cavity 12. In this method, an upper portion of the filling powder is pressurized downwards (in a direction indicated by an arrow A) by pressurizing means such as a leveling rod (not shown) disposed in the feeder box 13 for the purpose of suppressing feeding inconsistency.
According to such a conventional filling method using a feeder box, the powder can be surely filled in the cavity. In addition, the volume of the filling powder can be controlled to be substantially constant by means of the xe2x80x9clevelingxe2x80x9d done by the trailing edge of the feeder box.
Even in the case where the volume of the filling powder is constant, however, when a pressure applying to the powder by a leveling bar or the like is varied, the filling density is also varied. This eventually results in increased feeding inconsistency.
Moreover, according to the conventional method, the flowability of the powder becomes lower by a strong pressure due to the self-weight of the powder in a position closer to a bottom portion of the cavity, which leads to poor alignment of the powder in a magnetic field. As a result, as for the filling powder, the degree of alignment in a position closer to the bottom portion of the cavity is lower than the degrees of alignment in other portions. There arises a problem in that the magnetic properties are varied depending on positions.
This problem arises especially in the case where an Rxe2x80x94Fexe2x80x94B type rare earth magnet is to be manufactured. Rxe2x80x94Fexe2x80x94B type magnetic powder has a larger specific gravity as compared with ferrite magnetic powder. For this reason, in the case where the Rxe2x80x94Fexe2x80x94B type magnetic powder is filled in the cavity, a larger self-weight pressure is generated in a position closer to the cavity bottom portion, as compared with the case of the ferrite magnetic powder. Accordingly, in the case where the Rxe2x80x94Fexe2x80x94B type magnetic powder is used, the deterioration in powder alignment cannot be sufficiently suppressed even if the friction coefficient of powder particles is lowered by adding a lubricant to powder, or even if the intensity of the applied aligning magnetic field is increased. Thus, the magnetic properties of the final magnet product are likely to be degraded.
The invention provides a magnetic powder feeding method that can reduce a variation in degrees of alignment depending on the positions in the cavity.
The invention also provides a method for forming a compact with a uniform and high degree of alignment by using the magnetic powder feeding method, and a method for manufacturing a magnet with excellent magnetic properties.
The invention also provides a magnetic powder feeding apparatus that is suitably used in conjunction with the magnetic powder feeding method.
A magnetic powder feeding method of the present invention for feeding magnetic powder into a cavity of a pressing apparatus includes the steps of: placing the magnetic powder outside the cavity; forming a magnetic field in a space including the cavity; and moving the magnetic powder into the cavity using a force exerted on the magnetic powder by the magnetic field, while the magnetic powder is oriented in a direction of the magnetic field, wherein the step of moving of the magnetic powder into the inside of the cavity is performed after the start of the application of the magnetic field.
A magnetic powder feeding method of the present invention for feeding magnetic powder into a cavity of a pressing apparatus, includes the steps of: placing the magnetic powder outside the cavity; forming a magnetic field in a space including the cavity; and dropping the magnetic powder into the inside of the cavity by using an electromechanical mechanism that operates in an interlocking manner with the formation of the magnetic field.
In a preferred embodiment, a magnetic powder is moved into the cavity at a timing when an intensity of the magnetic field reaches a predetermined value.
In a preferred embodiment, the direction of the magnetic field includes a direction perpendicular to a pressing direction in the inside of the cavity.
In a preferred embodiment, the magnetic field is directed substantially in a horizontal direction in the inside of the cavity.
In a preferred embodiment, a member for preventing the magnetic powder from moving until the magnetic field is formed is inserted between the magnetic powder and the cavity, and after the magnetic field is formed, the member is driven, thereby enabling the magnetic powder to move.
A magnetic powder feeding method of the present invention for feeding magnetic powder into a cavity of a pressing apparatus includes the steps of: placing the magnetic powder above the cavity; forming an aligning magnetic field in a space including the cavity; and dropping the magnetic powder into the cavity while the magnetic powder is oriented in a direction of the magnetic field by using a force by which the aligning magnetic field attracts the magnetic powder.
In a preferred embodiment, the force exerted on the magnetic powder by the aligning magnetic field is in the same direction as the force of gravity on the magnetic powder.
In a preferred embodiment, the magnetic powder of an amount to be filled in the cavity is first placed above the cavity.
In a preferred embodiment, a direction of the aligning magnetic field includes a direction perpendicular to a pressing direction of the pressing apparatus in the inside of the cavity.
In a preferred embodiment, the aligning magnetic field is directed in a horizontal direction in the inside of the cavity.
In a preferred embodiment, a member for preventing the magnetic powder from dropping until the aligning magnetic field is formed is inserted between the magnetic powder and the cavity, and after the aligning magnetic field is formed, the member is driven, thereby enabling the magnetic powder to drop.
In a preferred embodiment, the step of placing the magnetic powder above the cavity is performed by using a powder vessel provided with an open and close mechanism that maintains a closed state by the weight of the magnetic powder, but can be downwardly opened by the force of the aligning magnetic field attracting the magnetic powder.
A method for producing a magnetic powder compact of the present invention includes: a step of feeding magnetic powder into a cavity of a pressing apparatus by the above-described magnetic powder feeding method; and a pressing step for compressing and compacting the magnetic powder fed in the cavity.
In a preferred embodiment, in the pressing step, the magnetic powder is compressed and compacted while the aligning magnetic field is continuously applied.
A method for manufacturing a magnet of the present invention includes: a step of feeding magnetic powder into a cavity of a pressing apparatus by the above-described magnetic powder feeding method; a pressing step for compressing and compacting the magnetic powder fed in the inside of the cavity, thereby producing a compact; and a step of sintering the compact.
In a preferred embodiment, in the pressing step, the magnetic powder is compressed and compacted while the aligning magnetic field that is applied in the step of feeding the magnetic powder into the cavity is continuously applied.
In a preferred embodiment, the aligning magnetic field applied in the pressing step has substantially the same intensity distribution as that of the aligning magnetic field applied when the magnetic powder is filled in the inside of the cavity.
In a preferred embodiment, at least in the cavity, the direction of the aligning magnetic field applied in the pressing step is the same as the direction of the aligning magnetic field applied when the magnetic powder is filled in the inside of the cavity.
In a preferred embodiment, the maximum magnetic field intensity of the aligning magnetic field applied in the pressing step is larger than the maximum magnetic field intensity of the aligning magnetic field applied when the magnetic powder is filled in the inside of the cavity.
A magnetic powder feeding apparatus of the present invention includes: a member for supporting magnetic powder against the weight of the magnetic powder; and means for dropping the magnetic powder upon the application of an aligning magnetic field.
A magnetic powder feeding apparatus of the present invention includes: a member for supporting magnetic powder against the weight of the magnetic powder; and means for performing an electromechanical operation coincide with the application of an aligning magnetic field, thereby dropping the magnetic powder.
A magnetic powder feeding apparatus of the present invention includes: a vessel for containing magnetic powder; a bottom plate that is pivotably supported by the vessel and on which the magnetic powder is placed; and open and close means for closing the bottom plate against the weight of the magnetic powder, wherein the open and close means pivots the bottom plate open when an aligning magnetic field formed by a pressing apparatus downwardly attracts the magnetic powder, thereby dropping the magnetic powder into a cavity of the pressing apparatus positioned below.
In a preferred embodiment, the vessel and the bottom plate are made of a nonmagnetic material.
A powder pressing apparatus of the present invention includes: the above-described magnetic powder feeding apparatus; and a die constituted by a magnetic material having a saturation magnetization of 0.05 to 1.2 tesla.